Playing with the idea of Signature Spells in various Vancian spell systems.
A Signature spell is one that once memorized can be cast up to three times per day.
I Am thinking that Magic-Users (and not clerics) gain 1 Signature Spell per spell level. So they can have a First Level Signature Spell, a Second Level One and so on. They could not gain this spell till the level after they gain the spell. So if a Magic-User wants to use Lightning Bolt as a signature spell, he would need to take the spell at 5th level (the first time he can take a 3rd level spell) and then make it his signature spell at 6th level.
He could not take lightning bolt at 5th and then make Fireball (another 3rd level spell) his signature spell at 6th.
I don't think it would terribly upset play.